[An abstract of a Lecture on Combustion delivered before the Mechanics' Institute, at Cincinnati, Ohio, by Prof. Chas. W. Wright.] By combustion is.commonly understood the chemical union of a combustible body with oxygen gas—the latter being called the sup porter of combustion; but this definition is in correct as a philosophical expression, for no one substance is a supporter ofcombustion, nor is any one intrinsically a combustible body. Thus, if a chandelier be suspended in one ot the large gasometers, at any gas manufactu ring establishment, and atmospheric air trans mitted through it, it can be ignited as it issues through the burners, and will burn in the same manner as ordinary coal gas does in the at mosphere, showing that coal gas will support the combustion of air. The proper definition of combustion is, therefore, the chemical com bination of two or more bodies with the evo lution of heat, and sometimes light. The terms combustible body and supporter of com. bustion, however, retained by chemical wri ters, and the use of them, though expressive of an erroneous idea, are convenient, and will not mislead if properly explained. If a body be volatile, its combustion is attended with flame, as sulphur, phosphorus, 'c.; if not volatile, the combustion, though attended with the evolution of heat and light, is not acaom- panied with flame, as charcoal, coke, iron, 'c. Most organic bodies burn with flame from the fact of their containing a large quantity of hy drogen, the most volatile body in nature. Ordinarily the combustion of a body in air is extremely rapid ; but sometimes it takes years for its completion. Thus, when iron is burned in oxygen gas, er the smith's forge, great heat and light are evolved, and the pro cess is soon completed; but in the rusting of the same quantity of iron in the air, an equal amount of he at is evolved, although it may take years to complete the operation. The heat being given out gradually in the latter case, s not taken cognizance of. The gradual combination of a body with oxygen is called “ low or slow combustion.” The rusting of all metals, as iron, zinc, 'c., is due to low combustion. Low combustion is more striking among organic bodies. Thus, the rancidity of fats and fixed oils, are instances of low com bustion. They become rusty, in fact, by com bining with the oxygen of the atmosphere, and undergoing partial combustion. Butter is more prone than most tats to this kind of change, and hence the most of it sold in cities is rancid or rusty. The decay of all organic matter is nothing more than its low combus tion. We have instances of it in the decay or rotting ofwood, where the same amount of heat is evolved as if it had been burned in an ordinary fire, but being given out slowly, it is not taken notice of. The conversion of the proto carbonate of iron into the peroxide, in the disintegration of rocks, is another instance of low combustion, and is the cause of their changing from a dark color to a light yellow. The quantity ot heat evolved, during the combustion of a body in air, is in proportion to the amount of oxygen consumed, and does not depend upon the amount of the combusti ble employed. Thus, one pound of oxygen in combining with charcoal, heats from 32° to 212° 29 pounds of water; and hydrogen, un der the same circumstances, heats 29pounds of water from 32° to 212°, so that it matters but little what combustible is used, the amount of heat evolved being regulated by the quanti ty of oxygen consumed. That substance is most valuable as fuel which will consume the largest amount of oxygen in a given time, and yields the smallest quantity of volatile product after combustion. The cause of the evolution of heat during combustion, has never been ex plained, and although numerous theories have been offered, and many experiments instituted to account for it, we know no more about it now than we did in the days of Lavoisier. The force that can be generated by combus tion is almost incredible ; thus, Sir J. Herschel has calculated that in the proper combustion of a bushel of coal, under a steam boiler, a force can be obtained sufficient to elevate 70,000,000 pounds weight a foot high, and yet the sun's rays exerted the same amount of force in de composing the carbonic acid absorbed by the leaves of the plants during the growth of their woody fibre, out of which the coal was form ed. Another interesting circumstance, con nected with wood, coal, peat, and all ordinary combustible substance, is the fact that they are the only bodies in nature that are fitted tor fuel, the only ones that pass off in invisi ble and in small quantities, harmless forms of matter—the only ones that are re-converted into the very same conditio that they occu pied before they were consumed. Combustion takes place only at the point where the bodies that burn are in contact this is best seen in an ordinary flame, as that of a candle. All common flames consist of a hollow luminous shell of light, the interior of which is filled with combustible gases, as may be seen by depressing a sheet of wire gauze over. them, which gives a section of the flame. Flame is composed of three distinct portions, each possessing different properties. The in ner dark portion of flame consists of a cone of vaporized combustible gases, which may be -drawn out by means of a small tube, and ig nited. Another portion, which may be de scribed as the middle cone, consists of little lumps of charcoal, or lamp-black, heated to whiteness. In this part of the flame the hy drogen ot the combustible gases alone burns, it having a greater affinity for atmospheric ox ygen than the carbon, combines with it, dis placing the lumps of carbon, which, from their high temperature, constitute the luminous part of flame. At the outer portion of flame ano ther cone may be observed where the light gradually disappears, and here the combustion is complete; the little lumps ot carbon being consumed, light is no longer evolved, although the temperature is higher in this part ot the flame than any other. The luminous part of flame, then, consists of particles of solid mat ter heated to whiteness. ' Gaseous matter can not be heated white-hot by the most intense degree of heat. The proper place for igniting a combustible body, in all ordinary flames is near the summit of the outer cone, where the temperature is highest, and the air in excess.